1. Field of the Invention
This invention relates to lithography systems for imaging patterns, and more particularly relates to lithography method and apparatus having a scan and repeat system characterized by complementary edge illumination by adjacent scans for producing precise images of a high resolution pattern from a mask onto a substrate at a high speed and over an image field substantially larger than the maximum field size of the imaging optics.
2. Description of the Prior Art
Lithography systems are extensively used in the production of integrated circuit chips and electronic circuit boards. Such systems typically include a primary exposure source such as a high intensity lamp or a laser or source of other radiation, mask and substrate positioning systems, a projection system to illuminate and image the pattern present on the mask onto the substrate, and a control system. The intent typically is to illuminate a wafer coated with a layer of a radiation-sensitive material so as to produce the desired circuit pattern, which later will be metallized or otherwise activated during further processing. Illumination may be by ultraviolet light or visible light or other radiation such as X-rays or electron beam. The desire is to illuminate the target regions selectively so as to activate a particular pattern. Integrated circuit chips typically undergo numerous illumination steps and physical treatment steps during production.
As the demand for chips with ever greater memory and processing capability increases, the individual bits on the chips get smaller in dimensions. This requires that the lithography equipment used for imaging these patterns have higher and higher resolution. Simultaneously, the larger physical size of the chips demands that the higher resolution be achieved over a larger image field.
An approach in the prior art to achieve high resolution is to use reduction optical imaging systems in which the pattern on the mask is reduced by a factor of 5-10 when reproduced on the wafer. Since such a reduction system is capable of high resolution only over a limited image field, the exposure region is confined to one chip size, approximately of an area 1 cm.sup.2. The entire wafer is processed by exposing a chip, stepping to the next chip, and repeating the process. In these prior art machines, known as step and repeat systems, the limiting performance capability is determined by the reduction projection lens assembly which typically consists of a large number of individul lens elements. As the resolution requirements increase, the design complexity of this lens increases. Further, if a lens is designed for a higher resolution, typically its image field size decreases. To design and build a lens with both higher resolution and larger field size is a task of great difficulty.
Another approach in the prior art has been to use an imaging system with 1:1 magnification ratio, in which the wafer is exposed through a long and narrow curved slit, and in which imaging over the entire wafer is achieved by scanning the whole wafer once across this long and narrow slit. Although such systems are capable of exposing large chips, they are severely limited in their resolution capabilities due to their small numerical aperture. Furthermore, as device resolution requirements shrink to submicron dimensions, and especially to 0.5 micron and below, such scanning systems suffer from the additional difficulty of requiring masks of the same high resolution as the devices to be produced. Consequently, these systems have not found much use for production of integrated circuit chips with submicron dimensions.
Another prior art to achieve a large image field is to use an imaging system known as Wynne-Dyson design which has a magnification ratio of 1:1. Although such systems can expose large chips, since their magnification ratio is 1:1, they suffer from the same disadvantages as stated in the above paragraph, namely, their resolution capability is severely limited because the mask requirements are made greatly more difficult.
Prior art lithography approaches employing electron beam illumination have used either a focused electron beam with a bit-by-bit serial writing process, or shadow projection through a 1:1 stencil mask. These systems suffer from the disadvantages of, respectively, low exposure speed and requirement of a complex and difficult mask technology. Prior art X-ray lithography systems have similarly used shadow printing through 1:1 membrane masks and thus, as discussed in the above paragraphs, have the same disadvantages due to 1:1 patterning and difficult mask requirements.
In view of the limitations in prior art as discussed above, there is an important need to develop a lithography system which delivers superior resolution, high exposure speed and greatly extended field size.